The present invention relates to a method in a real time control system for seismic interference.
In marine seismic surveys a relatively intense acoustic impulse is generated at regular intervals, for instance by use of so called air guns. Such guns thus generally emit impulsive acoustic signals. If a number of seismic surveys occur in a limited area, acoustic signals from each separate survey may be recorded by the seismic streamer of another survey and cause deteriorated data quality. When a shot is fired, the shot generated pressure waves are recorded at a plurality of measuring points or channels provided in a seismic streamer. The recording time of a certain reflector or point of reflection is determined by the distance of the measuring point from the source of sound, and the depth of the reflector. Recording may, however, be masked by simultaneous recording of pressure waves or reflection signals from another simultaneous seismic survey, and such recordings will overlay the signals from one's own survey and provide a seismic interference in the recording curves. The seismic interference will prevent detection of the reflections occurring in one's own survey. The recording time when this seismic interference occurs with the subsequent shot will depend on how much the simultaneously occurring seismic surveys are mutually out of step. Firing intervals generally being based on a fixed distance in the direction of movement, the degree of being out of step is determined by the velocity of one's own survey vessel and that of another vessel or other vessels, as well as by the course of the vessels.
A seismic streamer generally has a length of approximately 3000 m with an array of, e.g. 240 measuring points or channels, and a typical firing interval of 25 m. The common reflection points or CMP's in the Earth's crust are, thus, covered several times, i.e. by different measuring points or channels along the streamer with several shots. In case of a series comprising a plurality of shots which are fired at determined intervals with the vessel being in motion, reflections from the same CMP will be received, and the reflection waves will be received on different channels in the listening streamer. It is a fact well-known to those skilled in the art, that reflections from the same point in the Earth's crust, for instance the sea floor, are recorded at different measuring points along the seismic streamer for different shots. This phenomena is called coverage. In other words, coverage indicates how many different shots contributed to a series of recordings belonging to the same CMP or common reflection point. The separate recordings or measurements are summed after having been subjected to different, predetermined corrections or other data processing.
The amplitude of the recorded reflection signals is attenuated as a function of travel time. This is, inter alia, due to absorption along the longer path of propagation, and spherical dissipation. The above-mentioned seismic interference will, thus, be especially disturbing with increasing recording time and attenuated recorded amplitudes, and especially with an increasing level of interference relative to the level of amplitude.
According to prior art interference problems caused by a number of simultaneous seismic surveys are generally handled by interrupting data collection. Then an agreement is reached on a division of available survey time among different surveys. If the amplitudes of the interfering pressure waves are below a predetermined value, e.g. 20 .mu.bars and/or if the interfering sound waves have certain angles of incidence, e.g. from the rear, as opposed to the sound waves from one's own survey which has a frontal incidence, one's own acoustic source being towed in front of the seismic streamer, a predetermined level of interference may in some cases be accepted. If not, it will be necessary to interrupt one's own seismic survey and valuable production time is lost.
The great disadvantage of carrying out a seismic survey in an interference loaded environment is due to the fact that there is an element of uncertainty as to how much interference may be accepted as a function of the amplitude angle of incidence of the interference signal, and the shot frequency in relation to geological targets of the seismic survey. Additionally, necessary information to make a proper decision as to an acceptable level of interference will generally not be available. Other kinds of information are, thus, necessary to make a realistic assessment of the interference problem. At the same time there is also the problem that often there are no operators present on the survey vessel who are qualified to make a comprehensive assessment when interference phenomena occur, and on the basis of information available according to prior art.
Consequently, a conservative attitude often prevails as regards accepting interference. In practice, this means that collection of data is generally interrupted and an agreement is reached about division of available production time among the various surveys. This will obviously result in loss of valuable survey time for parties involved.